A compression-type refrigerator is generally comprised of at least a compressor, a condenser, an expansion mechanism (such as an expansion valve) and an evaporator or comprised additionally of a dryer, and is constructed such that a mixed liquid composed of a refrigerant and a lubricating oil (refrigerator oil) is circulated through a closed system including these devices. In such a compression-type refrigerator, in general, a high temperature is established within a compressor while a low temperature is established within a cooler, though depending upon the devices used. Therefore, it is necessary that the refrigerant and the lubricating oil can be circulated through the system without causing a phase separation in a wide temperature range encompassing from low temperatures to high temperatures. In general, the refrigerant and lubricating oil have temperature regions on a low temperature side and on a high temperature side in which they cause phase separation. The highest temperature in the low temperature side separation region is preferably −10° C. or lower, particularly preferably −20° C. or lower. On the other hand, the lowest temperature in the high temperature side separation region is preferably 30° C. or higher, particularly preferably 40° C. or higher. Phase separation during the operation of the refrigerator considerably adversely affects the service life and the operation efficiency of the apparatus. For example, if phase separation between the refrigerant and lubricating oil occurs in the compressor section, lubrication in a moving part will become insufficient so that seizing and other problems will occur to considerably reduce the service life of the apparatus. On the other hand, if phase separation occurs in the evaporator, the heat exchange efficiency is considerably reduced because of the presence of a highly viscous lubricating oil.
As a refrigerant for refrigerators, a chlorofluorocarbon (CFC), a hydrochlorofluorocarbon (HCFC), etc. have been hitherto mainly used. Because these compounds contain chlorine atoms which cause environmental problems, chlorine-free substitutional refrigerants such as a hydrofluorocarbon (HFC) have been investigated. Thus, hydrofluorocarbons, typically 1,1,1,2-tetrafluoroethane, difluoromethane, pentafluoroethane, 1,1,1-trifluoroethane (which are referred to as R134a, R32, R125 and R143a, respectively) have become a focus of attention. For example, R134a is actually used in a car air conditioning system.
Since there is apprehension that HFC may also have a problem with respect to global warming, however, a natural refrigerant such as carbon dioxide has become a focus of attention as a substitutional refrigerant. Because carbon dioxide requires a high pressure, however, it is impossible to use carbon dioxide in current car air conditioning systems.
As a refrigerant having a low global warming potential and being usable for a current car air conditioning system, there has been found a refrigerant having a specific polar structure in its molecule, such as an unsaturated fluorinated hydrocarbon compound (see, for example, Patent Document 1), a fluorinated ether compound (see, for example, Patent Document 2), a fluorinated alcohol compound or a fluorinated ketone compound.
A lubricating oil for refrigerators using such a refrigerant is required to have excellent compatibility with the refrigerant, good sealing properties, capability of imparting a low coefficient of friction to sliding members and, yet, excellent stability.    [Patent Document 1] Japanese Translation of PCT International Application Publication No. 2006-503961    [Patent Document 2] Japanese Translation of PCT International Application Publication No. H07-507342